The present invention relates to an improved tow coupling apparatus, for use between prime movers, such as trucks, automobiles, four wheel drive vehicles, tractors, farming equipment, motor cycles, and their trailers or towed equipment.
Generally, tow couplings are designed to allow a certain range of pivotal movement about three orthogonal axes depending upon the design of the tow coupling. There are many tow couplings presently being marketed, but two major types are predominant. They are, firstly, the various ball type couplings, and secondly, the various swivel ring and pintle hook combinations.
Although each of the alternative designs of such tow coupling arrangements operate in a variety of ways, it is apparent that the easiest design criteria to satisfy has been that of the load requirements of the tow coupling, especially those of the ball type.
The limited pivotal movement of such known ball type couplings is shown in FIGS. 1 and 2. Due to the configuration of such vertically disposed ball coupling arrangements the neck connecting the ball prevents the detachable coupling body rotating more than approximately 11 degrees in any direction relative to the horizontal plane. This limitation has been imposed by the standards set down by the Standards Association of Australia in its publication "Ball Couplings for Automotive Purposes", published in 1968. The standards specify minimum neck diameters as a strength requirement.
In practice it has been found that drivers of motor vehicles towing trailers frequently attempt to negotiate dips and humps wherein the trailer may need to rotate more than this allowable 11 degrees, either laterally or longitudinally relative to the longitudinal axis of the towing vehicle. Where this limitation of the coupling is exceeded, stresses and strains are imposed upon the vehicle, the trailer and the towing device. Damage to the vehicle, the trailer or the towing device is therefore often sustained in such circumstances. The towing device is often broken completely.
A modification to the detachable ball coupling body can overcome the lateral rotational limitation of about 11 degrees of the simple ball coupling. Fabricating the detachable ball coupling body with a longitudinal shaft rotatably attached to the drawbar of the trailer is an alternative practice. However this modification does not overcome the major disadvantage of all ball type couplings.
All ball type couplings suffer the principal disadvantage of being unable to permit the trailer to make a longitudinal vertical plane inclination of more than about 11 degrees plus or minus relative to the longitudinal axis of the towing vehicle as shown in FIG. 1, without causing overt stresses or strains or damage to the towing system.
The swivel ring and pin or swivel ring and pintle hook combinations also have major disadvantages. They are usually used where either, the trailer is very heavy, or the trailer must be capable of being inclined or declined more than approximately 11 degrees relative to the longitudinal axis of the towing vehicle.
The swivel ring coupling is used on agricultural equipment, heavy trailers used typically by statutory authorities, shire councils and contractors, and by trucking firms. The disadvantages of the swivel ring and pintle hook coupling are major. The coupling is necessarily loose which results in annoying rattling. Also, the head of the trailer drawbar moving laterally relative to the longitudinal axis of the towing vehicle, results in the trailer "wandering" from side to side whilst the prime mover and trailer are in motion. That is, the swivel ring necessarily cannot prevent the track of the trailer varying from that of the towing vehicle.
Ball type couplings are by far the most commonly used, especially on light applications such as domestic trailers, boat trailers and holiday caravans. A number of alternative designs marketed purport to overcome the principal disadvantages of both ball type couplings and swivel ring combinations, but, in practice, are generally found to be lacking the qualities that customers require. The qualities that the market demands of a tow coupling include,
(i) a much larger degree of pivotal movement than that provided by ball type couplings,
(ii) ease of coupling and uncoupling,
(iii) freedom from annoying rattling, and
(iv) accurate straight line tracking of the trailer.
The present invention seeks to overcome or substantially ameliorate the limitations and disadvantages of the prior art.